A coat of boron to capture impurities
Impurities, in the form of particles detached from materials inside the vacuum vessel, can be among a plasma's worst enemies. Even in trace amounts, they drain energy from the plasma, degrade its performance and lead in some cases to its complete collapse. In fusion machines, contamination of the plasma by impurities is mitigated two ways: at their source by implementing specific first-wall materials and "wall conditioning," and by preventing their penetration into the plasma through specific exhaust mechanisms such as divertors and cryopumps.
No technological option is a panacea and choosing tungsten over beryllium implies a whole series of adaptations, modifications and, in some areas, the implementation of new processes. Experience accumulated from dozens of operating tokamaks has shown, for instance, that tungsten is not a good oxygen getter. How to make it better and more efficient? Thirty-five years ago, after close to ten years of research, an innovative technique called "boronization" was developed for the Textor tokamak by Professor Jörg Winter and his team at the Forschungszentrum Jülich research centre in Germany.
Boronization has been applied successfully on practically every fusion device since, with the exception of JET (which used beryllium as first-wall armour material).
A few weeks ago, Professor Winter was at ITER to present the principles and challenges of the technique he pioneered. "Boronization," he explained, "is the racehorse of wall conditioning. It is a plasma-chemical process that consists in coating the entire plasma-facing surface of a fusion device with a thin (~10-100 nanometres) boron layer."
It consists in creating an electrical flow in a conductive environment, and using it to deposit particles on an object. The current originates from an electrode (an anode) with the object to be plated acting as a cathode.
In the "glow discharge" process used for boronization, anodes are used to create the electrical current flowing to the wall of the plasma chamber, which acts as a cathode. Once the magnetic field has been turned off, a gas containing boron is injected into the vacuum vessel chamber and submitted to a "very mild" electrical discharge, like in a neon light, resulting in the gas acquiring the properties of an electrically conductive plasma. The ionized particles it contains are accelerated by the electrical field and, as their molecular bonds break upon impact with the wall, boron is deposited as a perfectly homogeneous, "atomically clean" film on the wall's surface. The technique is "easy to implement," says Prof. Winter.
Once boronization is complete after 5 to 10 hours of continuous controlled glow discharge, particles—and particularly oxygen—are trapped under or within the boron film and cannot be released into the plasma.
Boronization creates optimal conditions for starting up plasmas. But as time passes and plasma discharges accumulate, the boron film becomes less efficient. On average, boronization is repeated every few weeks, "but if you do a good job you can significantly extend the interval between two boronizations," according to Prof. Winter.
